During the early part of this century it was established that sunlight exposure was essential for the prevention and cure of rickets. Intensive investigations led to the concept that when skin is exposed to the UV-B protion of sunlight the 7-dehydrocholesterol stores partly convert to vitamin D3. However, very little is known regarding the sequential events that occur when cutaneous 7-dehydrocholesterol converts to vitamin D3. Our initial investigations into the photobiology of vitamin D3 have suggested that the skin is a unique organ responsible for the controlled synthesis and release of this important calcium hormone. We have established that during exposure to sunlight, epidermal 7-dehydrocholesterol is converted principally to previtamin D3 and not directly to vitamin D3. Once formed in the skin, this thermally labile previtamin slowly converts to vitamin D3 over a period of days. The vitamin D binding protein which has preferential affinity for vitamin D3 transports vitamin D3 through the circulation leaving the previtamin behind to continue its conversion to vitamin D3. During the past three years we have developed the necessary methodology including (a) synthesis of radiolabeled 7-dehydrocholesterol (b) non-cytotoxic skin separation techniques and (c) a powerful high resolution high pressure liquid chromatographic system that directly analyzes the photoconversion of 7-dehydrocholesterol to previtamin D3 in human skin. Using these techniques we plan over the next five years to critically investigate the photobiology of vitamin D3 regarding (1) production rates of previtamin D3 as a function of wavelength and irradiance, (2) the exact location(s) for previtamin D3 synthesis, (3) the possible role of melanin on controlling cutaneous previtamin D3 formation, and (4) an investigation to evaluate the therapeutic efficancy of producing vitamin D3 metabolites on the skin by combining a topical application of a hydroxylated form of 7-dehydrocholesterol with UV-B phototherapy.